(1) Field of the Invention
The invention is related to the field of air and hydroacoustic transducers and in particular to piezoelectric shear transducers.
(2) Description of the Prior Art
Distortional piezoelectric devices have been used in a variety of circumstances where detection of air and water-borne acoustic energy is required.
In particular, piezoelectric transducers have been widely used as a part of sonar systems. When the sonar is in active mode, an AC voltage is applied to a piezoelectric transducer. Each voltage change causes the piezoelectric transducer to generate a mechanical response. Rapidly changing voltage, (such as an AC cycling), causes a mechanical response to occur multiple times a second which can then be amplified to produce an acoustic. wave. This wave is the sonar "ping" used to detect targets in active sonar.
In passive mode sonar, the transducer is not used to transmit acoustic signals, but is, instead, used to receive incoming acoustic waves. In this mode, acoustic waves cause mechanical movements in the piezoelectric transducer which are converted to electrical signals. The electrical signals can then be processed by sonar displays, computerized tracking devices, and other electrical processors. The detection mode of the transducer is also used in active sonar to detect reflected sonar pulses. Often, the same transducer is used both to generate pulses and receive reflected signals.
A major design goal of piezoelectric transducers when used as an element of sonar systems has been to provide high sensitivity. Higher sensitivities allow the sonar system to detect and respond to lower energy return signals. Additionally, optimal transducers must generate and respond to a wide frequency range of signals. Furthermore, the transducer also must be easily configured for installation in a variety of devices.
Piezoelectric devices have three main modes of operation: a cross-sectional compression mode (33 mode), a lateral compression mode (31 mode) and a shear mode (15 mode). These three modes are depicted in FIGS. 1a-1c, respectively. In 33 mode, a circular disc is polarized so that the flat surfaces carry opposing charges. Piezoelectric response comes from the compression and expansion of the disc up and down toward the two polarized surfaces. In 31 mode, a circular disc is again polarized so that the flat surfaces carry opposing charges. Piezoelectric response now occurs due to expansion and compression of the disc in a direction perpendicular to the axis going through the charged surfaces, resulting in a movement of the entire disc edge in and out. In the final mode, 15 mode, the disc is again polarized so that the flat surfaces carry opposing charges. However, since the surface on the bottom end is now held in a fixed location and the top flat surface is allowed to move laterally, a shearing of the disc results, thereby generating an electric response. The mode numbers correspond to the piezoelectric coefficients d.sub.15, d.sub.31, and d.sub.33. Research in piezoelectrics has shown that the shearing coefficient (d.sub.15) is much larger, indicating a more efficient response to incident acoustic energy than either of the other two coefficients.
However, previous attempts have focused on the amplification of the first two modes of piezoelectric response. In large part, this has been so because the amplification of the shear piezoelectric response has been more difficult to achieve. Only in the first two modes has amplification been easily made possible. The use of the first two modes (often called dilatational piezoelectric devices) has resulted in transducers with a more limited sensitivity and higher frequency than could have been achieved by using 15 mode (distortional) piezoelectrics. Furthermore, these devices require a plane of symmetry to allow free movement of the transducer element, thus limiting the geometry of devices in which they may be installed.